Motor vehicles with internal combustion engines conventionally employ a fan to move cooling air through one or more heat exchangers to reduce the temperature of fluids such as engine coolant, engine oil, automatic transmission fluid and air conditioner refrigerant. Fan speed is typically controlled by a fan drive having a clutch with a driven input member rotating at an input speed and an output member attached to the fan rotating at the fan speed. Fan drives that operate to provide variable or controllable fan speeds are known in the art. Typically, such fan drives vary the amount or the viscous properties of a contained fluid to vary the angular fan speed by transferring torque between the input member and the output member. For example, in a magnetorheological (MR) fan drive, an MR fluid is carried in a working chamber within the fan drive. Selective application of a magnetic field controls rotation of the output member relative to the driven input member by varying the effective viscosity of the MR fluid. MR fluids are known in the art, and typically comprise ferromagnetic particles, such as finely powdered iron or iron alloy, dispersed within a carrier fluid, such as mineral oil or silicone. MR fluids transform almost instantaneously into a semi-solid state in the presence of a magnetic field as the ferromagnetic particles assemble into somewhat stiff chains along the magnetic field, thereby increasing the torque transferred between the input member and the output member. When the magnetic field is removed, the chains disperse and the MR fluid re-liquefies, such that the fan drive input and output members are only minimally engaged, permitting relative and significantly independent rotation therebetween. Therefore, varying the strength of the magnetic field provides rapid, variable control over fan speed. However, the slippage in the fan drive clutch does result in slip power generation and loss, which can translate into clutch temperature increase. It is desirable to limit, if possible, the clutch temperature for several reasons, fan drive durability being one of them.